Large generators and motors are routinely examined for laminate shorts and stability when stationary. Various methods are available for this purpose.
One of the methods to determine laminate shorts comprises the magnetization of the entire laminated body by means of an auxiliary coil at the mains frequency, and the measurement of stray fields on the inner surface of the stator bore. The magnetization is carried out to relatively low values of the magnetic induction, typically to about 10% of the normal operating induction. This method of measurement is also known by the name “low-induction laminate short measurement” or by the name “ELCID” (electromagnetic core imperfection detector).
By way of example, U.S. Pat. No. 4,996,486 describes one method of this type. The prior art is therefore for the laminated stator core to be magnetized by means of an auxiliary coil and a sinusoidal auxiliary voltage applied thereto at the mains frequency, to about one tenth of the operating induction. This auxiliary voltage is normally derived directly from the mains voltage. An electrical recording coil is then moved away from the surface of the stator bore, with the recording coil being located close to the surface of the laminated core.
The currents which flow as a result of the interlaminar short circuits in the laminated core now induce voltages with a characteristic phase angle and amplitude magnitude in the recording coil. The characteristic phase angles and amplitudes make it possible to distinguish between points where there are laminate-short currents and points where there are no laminate-short currents. It is therefore possible to locate laminate shorts, and to assess the magnitude of the short-circuit currents, by means of this stray-field recording coil.
The invention provides alternative solutions to determine laminate shorts and the stability of a stator core.